1. Field of the Invention
The present invention relates to high power lasers, and more specifically, it relates to diode-pumped alkali vapor lasers.
2. Description of Related Art
The largest market currently for high-power ( greater than kilowatt) lasers is for the materials processing applications of sheet-metal cutting and welding [1].
Application end-users continue to call for multi-kilowatt lasers with near-diffraction-limited output beam quality, wavelengths of  less than 1060 nm, higher efficiency and compactness, and decreased cost-of-ownership, compared to traditional lamp-pumped Nd:YAG solid state lasers and electrically-pumped CO2 gas lasers [2].
The near-diffraction-limited (i.e., M2 less than 2 or 3, where M2 is the times-diffraction-limit factor) feature provides for greater intensity on the work-piece, for a focusing lens having a given f/number. Shorter wavelengths in general result in enhanced absorption efficiency in most metals and allow for power delivery to the work piece by optical fiber. Higher efficiency and compactness generally lead to low cost of ownership.
The direct use of high-power 2-D laser diode arrays for material processing applications has been of great interest in the past few years [3]. However, the output beam of a high-power (60 watt) linear bar array of laser diodes is typically  greater than 1000 times the diffraction limit, and that of a 2-D stack of bar array is typically more than several thousand times the diffraction limit [4]. Spectral widths of diode arrays are typically several nm wide. Some applications, such as spin-polarizing Xe gas for use in medical applications [4], could benefit from having available diode arrays with considerably narrower spectral widths. Efforts continue to narrow spectral width [5] and improve beam quality [6] of 1-D and 2-D laser diode arrays, but cost effective methods appear to be complex and expensive.
An alternative means of effectively improving the beam quality and/or spectral width of highly multi-mode 1-D and 2-D laser diode arrays is to use them to pump another laser, whose output beam can be extracted in a low order spatial mode (e.g., near-diffraction-limited, or M2=2 or 3), and with a greatly reduced spectral width. In effect the pumped laser becomes a xe2x80x9cspatial and spectral brightness converterxe2x80x9d, trading a small loss in energy efficiency for a much greater gain in beam quality and spectral narrowness. The diode-pumped solid-state laser (DPSSL) is such a brightness converter. Nd:YAG DPSSLs have been developed recently that exhibit increased efficiency and beam quality compared to traditional lamp pumped devices [4].
Notwithstanding the reduced (xcx9c⅓) thermal loading realized by diode pumping (compared to lamp pumping), practical, near-diffraction-limited, multi-kilowatt Nd:YAG DPSSLs have remained elusive because of severe thermally induced focusing and stress-birefringence [7] present in solid state laser hosts, such as YAG.
In light of the foregoing, the need continues for a cost-effective solution for an efficient, compact, multi-kilowatt, near diffraction-limited, narrow-spectral-band laser source emitting at wavelengths  less than 1060 nm.
It is an object of the present invention to provide an alkali vapor laser.
It is an object of the present invention to provide an alkali vapor laser having laser diode array pumping mechanism for pumping an optical cell that include an alkali vapor selected from the group consisting of cesium (Cs), rubidium (Rb), potassium (K), sodium (Na), and lithium (Li).
Another object of the invention is to include a buffer gas in an alkali vapor optical cell, where the buffer gas is selected from the group consisting of rare gases and light molecular gases.
Still another object of the invention is to include in an alkali vapor optical cell the rare gases of xenon, krypton, argon, neon, and/or helium.
An object of the invention is to include in an alkali vapor optical cell the light molecular of hydrogen, methane, ethane, propane, and/or their deuterated analogues.
An object of the invention is to provide a means for off-axis coupling of linearly-polarized radiation from a pump laser into an alkali vapor gain medium, where the linearly-polarized radiation couples into the gain medium without passing through the input mirror.
Another object of the present invention is to provide an alkali vapor laser where laser radiation at wavelength xcex1 is generated in a linear polarization orthogonal to the pump radiation.
Another object is to provide a method for converting spectrally broadband radiation from a pump semiconductor diode laser array into spectrally narrowband output laser radiation from an alkali/buffer-gas gain mixture.
Still another object is to provide a method for converting the substantially divergent, multi-spatial-mode of semiconductor diode laser array pump radiation into a near diffraction-limited, near-single-spatial-mode, coherent laser radiation from an alkali/buffer-gas gain mixture.
These and other objects will be apparent to those skilled in the art based on the teachings herein.
The present invention provides an efficient, compact, high-power, near diffraction-limited laser source emitting at a wavelength  less than 1060 nm. The invention is a new class of lasers that can be pumped by conventional high-power, multi-mode, broadband 1-D and 2-D laser diode arrays, where the pumped laser gain medium comprises an atomic vapor of one the alkali elements (Li, Na, K, Rb or Cs), buffered with a mixture of rare-gas (He, Ar, Kr, Ne or Xe) and selected molecular gases. Given the central role of the alkali atomic vapor as the active laser entity, this new type of laser is herein designated as the diode-pumped alkali laser (DPAL).
The three lowest-lying electronic levels of the alkali atom are utilized in the present DPAL designs, which is a classic xe2x80x9cthree level laserxe2x80x9d. In the DPAL laser, the alkali atom gain medium is pumped at a wavelength matching the wavelength of the 2S1/2-2P3/2 electric-dipole-allowed transition (the D2 transition). After kinetic relaxation of pump excitation to the excited 2P1/2 electronic level, laser emission takes place on the 2P1/2-2S1/2 transition (the D1 transition).
In DPAL operation, pump radiation centered at the pump wavelength xcexp of the D2 transition, is directed into a gain cell containing alkali atoms and buffer gases. The alkali atoms in the gain cell are selectively pumped to the D2 transition, whereupon they collisionally relax to the lower-lying D1 transition before they can radiatively decay back to the ground level. The buffer gas also serves to collisionally broaden the alkali D-transitions.
The D2 transitions for Cs, Rb, and K lie in the spectral region (760-850 nm) for which powerful and efficient high power laser diode arrays are commercially available. Therefore, these particular alkali atoms are utilized in preferred DPAL embodiments.
A basic DPAL device configuration takes the form of an xe2x80x9cend-pumpedxe2x80x9d configuration, accommodating the fact that a DPAL is a true three-level laser. In these designs the DPAL active medium is contained within a cell, which is fitted with flat optical windows at either end to contain the alkali atomic vapor. The window at the pump end of the apparatus is coated on the exterior surface with a multilayer dielectric stack to form a mirror of the laser cavity. This mirror coating provides high transmission at the pump wavelength xcexp and high reflectivity at the laser wavelength, xcex1. The window at the other end of the cell (away from the laser diode pump array and pump light coupling lens) is coated on its exterior surface with an anti-reflection layer for both pump and laser wavelengths. The laser cavity is completed with a second mirror placed along the axis of the gain cell. This mirror is configured to permit only the fundamental (or other desired low-order) spatial mode of the resonator to oscillate. The output mirror is coated to have a high reflectivity at the pump wavelength, to reflect pump radiation that was not absorbed during a first pass through the cell to return generally parallel to the cell axis for a second pass. The coating on the output mirror is also designed to provide a reflectivity at the laser wavelength that optimizes the output coupling of laser radiation generated within the gain cell, and maximizes the efficiency of the DPAL.
To energize the DPAL, pump radiation provided by a laser diode pump array having a wavelength centered at the D2 transition is coupled by a lens into the gain cell generally along the cell axis, through the end mirror on the cell, and double-passed through the cell following reflection from the high reflectivity mirror placed at the other end of the gain cell. Laser radiation generated within the gain cell at the wavelength matching the wavelength of the D1 transition is extracted through the partially transmitting output mirror.
An alternative preferred DPAL embodiment has a thin-film polarizer that is inserted between the diode pump array and the alkali gain cell. The cell windows are AR coated on their exterior surfaces to maximize transmission at pump and laser wavelengths. Polarized pump radiation is coupled into the apparatus by passing through the thin-film plate polarizer with high transmission, and is focused within the cell to provide good spatial overlap with a low-order spatial mode of the laser cavity. The laser cavity is formed between the highly reflecting (at pump and laser wavelengths) mirror and the output coupling mirror. Laser action in the pumped cell is set up in a polarization perpendicular to that of the pump radiation due to the presence of the thin-film plate polarizer within the laser cavity.
In another DPAL embodiment, the radiation from a 2-D laser diode pump array is coupled into the gain cell using a hollow lens-duct. An unstable laser cavity is formed by a dot-mirror placed in the center of a cell window. An anti-reflection coating is placed on the cell window in the annular region surrounding the high-reflectance dot mirror. Pump radiation is coupled into the gain cell in this annular region and propagates through the cell reflecting from a mirror coating placed on the outer barrel of the transparent-walled cell.
Many other embodiments for DPAL type devices will be obvious to those skilled in the art based on the teachings herein. Spectrally narrow laser operation can be further enhanced by incorporating a birefringent filter (BRF) within the laser cavity. Unexpectedly high pump power absorption efficiency can be realized in a DPAL device because it proves possible to effectively couple much of the pump power into the alkali atoms through the Lorentzian wings of the pump transition.